netdev - Re: [PATCH 1/2] net: Toeplitz library functions

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Date:	Tue, 24 Sep 2013 11:02:11 -0700
From:	Tom Herbert <therbert@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	David Miller <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>,
	"Brandeburg, Jesse" <jesse.brandeburg@...el.com>
Subject: Re: [PATCH 1/2] net: Toeplitz library functions

On Tue, Sep 24, 2013 at 10:34 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Tue, 2013-09-24 at 10:03 -0700, Tom Herbert wrote:
>> On Tue, Sep 24, 2013 at 9:46 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> > On Tue, 2013-09-24 at 09:35 -0700, Tom Herbert wrote:
>> >
>> >> We should really be using rxhash for that anyway, eliminate this
>> >> ehashfn.  This would entail adding rxhash argument in the various
>> >> udp_lookup functions.
>> >
>> > Nope : Some NICs provide UDP rxhash only using L3  (source IP,
>> > destination IP), not L4 (adding source & destination ports)
>> >
>> Then the NIC won't set l4_rxhash and we'll rehash over 4-tuple when
>> skb_get_rxhash is called.
>
> Yes, but then in this case you add cpu cycles for no reason.
>
> If you have multiqueue NIC, you do not use RPS/RFS, so skb->rxhash might
> be 0
>
> hash = inet_ehashfn(net, daddr, hnum, saddr, sport);
>
> is faster than the whole flow dissection game.
>
But if we already have valid l4_rxhash we're just wasting time
recomputing it (very like the same value).  So just do:

if (skb->l4_hash)
    hash = skb->rxhash
else
    hash = inet_ehashfn(net, daddr, hnum, saddr, sport);

Even if we go the inet_ehashfn route in the packet, it's makes sense
to store this in skb->rxhash so that subsequent functions don't need
to compute the hash

By the way, I believe there is an insidious in using the same hash
value or even related hash values in multiple places for steering.
Since we typically do something like hash % numqueues, we introduce
bias if multiple steering levels look at the same bits.  With
SO_REUSEPORT this might actually be beneficial since we could get port
selection to match RSS locality, but this is not by design!

>
>
>
>
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